1. Field of the Invention
The present invention relates to a voltage-controlled oscillator (hereinafter referred to as VCO) circuit and a phase-locked loop (PLL) circuit using the VCO circuit.
2. Description of the Related Art
FIG. 1 is a block diagram showing a PLL circuit described in JP 06-152401 A (FIG. 1). A three-input one-output switch denoted by reference numeral 4 receives an output signal from a loop filter 3 and two reference voltages VREF1 and VREF2 different from each other and selects one of three inputs in response to a control signal from a calibration circuit 5.
A VCO 2 includes a voltage/current converter (hereinafter referred to as V/I converter) 2A for converting a voltage value “V” into a current value “I”, a current-controlled oscillator (hereinafter referred to as ICO) 2B for oscillating a signal having a predetermined frequency in response to an output of the V/I converter 2A, and a gain regulator (denoted by VR in FIG. 1) 2C. The V/I converter 2A receives an output signal from the switch 4 and outputs a signal obtained by V/I conversion to the gain regulator 2C. The gain regulator 2C regulates a value of a current flowing from the V/I converter 2A to the ICO 2B in response to a signal which is based on a gain set value M and outputted from the calibration circuit 5.
According to the PLL circuit which has the structure described above and is manufactured as an integrated circuit (hereinafter referred to as IC), process variability can be compensated, so a loop band can be made constant.
However, the conventional technology described above has a problem in that it is necessary to evaluate variability of manufactured ICs and specify the gain set values M based on the variability.